This disclosure generally relates to a thermoplastic resin molding composition comprising a polyorganosiloxane-polycarbonate block copolymer, polycarbonates, and a flow promoter for producing articles having improved surface aesthetics.
Aromatic polycarbonate polymers have found wide use as engineering thermoplastics, having inherent toughness and clarity. However, there is a need to improve the low temperature impact strength and ductility of the polycarbonate polymers. At low temperatures, brittle fractures are often a problem. Many variants of polycarbonate polymers have been evaluated to overcome these deficiencies.
One such variant is the polyorganosiloxane-polycarbonate block copolymers. These copolymers possess enhanced properties such as low temperature impact strength, low temperature ductility, improved processibility, and fire retardance. The polyorganosiloxane-polycarbonate block copolymers can be advantageously blended with various proportions of aromatic polycarbonate polymers to prepare thermoplastic molding compositions capable of meeting the wide range of requirements for high and low temperature properties. These blends also exhibit other useful advantages, such as ease of achieving fire retardance. By varying the selection of the polycarbonate polymer and the proportions of blend ingredients, a wide range of properties may be obtained, starting with a single polyorganosiloxane-polycarbonate block copolymer. However, when such blends are used to produce molded articles, the articles display inferior surface aesthetics, thereby limiting their utility in a variety of applications where the blends"" superior properties could otherwise be used advantageously. Hence there is a need for developing thermoplastic polymer molding compositions, which show improved surface aesthetics while also retaining the other properties, such as low temperature ductility and fire retardance.
Disclosed herein is a thermoplastic polymer molding composition comprising a polyorganosiloxane-polycarbonate block copolymer and a flow modifier, wherein said flow modifier is selected from the group consisting of a polyalkylene glycol compound, a low weight average molecular weight polycarbonate polymer, or mixtures thereof. The amount of flow modifier added should be an amount effective to reduce surface imperfections if the molding composition is molded into an article. Surface imperfections may be minimized by adding the correct amount of flow modifier. The correct amount can be determined by experimentation as shown in the Examples.
Another aspect of the disclosure is an article comprising a molding composition, wherein said molding comprises: a polyorganosiloxane-polycarbonate block copolymer; at least one polycarbonate polymer; and a flow modifier; wherein said article has a percent improvement in the shift in color index at surface imperfections of at least 10 percent relative to an otherwise similar molding composition which does not comprise said flow modifier.
Another aspect of the invention is a thermoplastic polymer molding composition useful for producing an article, wherein each 100 parts of said molding composition comprises from about 0.2 parts to about 5 parts of a flow modifier; wherein said polyorganosiloxane-polycarbonate block copolymer comprises: polyorganosiloxane blocks having the formula: 
wherein R1 is hydrogen, methoxy or allyl, xe2x80x9caxe2x80x9d is an integer having a value from about 40 to about 55; and polycarbonate blocks having the formula: 
wherein said flow modifier causes minimized surface imperfections in said article.
The above-described embodiments and other features will become better understood from the detailed description that follows.